How We Operate Our AMS

The 14CHRONO Centre operates a NEC compact model 0.5MV AMS
for carbon dating.
Installation of the machine began in January
2007 in a purpose built laboratory.
We typically measure about 200 unknown samples and 30 known age
secondary standards per month. Sample preparation, pretreatment, graphitisation
and analysis is carried out by a team who,
among them, have over 100 years of radiocarbon dating experience.

We have two graphite production rigs, one based on a
zinc reduction
and the other on hydrogen reduction.
Each rig has 12 reactors, and can be run once a normal workday.
Sample wheels going into the AMS will generally contain 3 days
production from one of the rigs.
We do not mix graphite produced under hydrogen with graphite
produced under zinc in a single sample wheel.

The structure of our sample wheels is the following:
A typical wheel has 4 rungroups: a tuning rungroup followed by 3 rungroups
of eight unknowns. The tuning rungroup consists of two blanks, two OX2 standards,
and 2 pairs of secondary standards. The blanks are usually combusted coal, for
which we get a 14C / 12C ratio of 1.8-2.5 e-15,
corresponding to a radiocarbon age of about 50,000 years.
The secondary standards are mainly taken from among

Identifier

Comment

F14C

TIRI/FIRI C

International Intercomparison

0.079

UB4388

radiometrically dated Irish oak

0.838

TIRI/FIRI D

International Intercomparison

0.422

TIRI/FIRI I

International Intercomparison

0.421

ANU sucrose

IAEA-C6

1.506

Each cathode in the tuning rungroup is run for four 2-minute exposures.
A typical exposure of the OX2 standards produces approximately 45,000 counts.
If the AMS machine is deemed to be running satisfactorily at this point, the
tuning rungroup is transferred to the last rungroup where all cathodes will be
run for 4 more exposures at the end. All cathodes are run for seven 2-minute
exposures.

The first and last rungroups of unknowns 'borrow' an OX2 from the tuning
rungroup, so that the wheel contains 6 OX2 standards.
If all the OX2 standards are sufficiently alike, the unknowns in each rungroup
are normalized to the nearest in time runs of bracketing OX2 runs to form
the ratio to standard. If one or more of the OX2 standards are deemed to be
outliers, an average of all the standards is taken for ratio to standard
normalization.